This invention relates to an integrated-circuit relaxation oscillator and more particularly to such an oscillator that is capable of operating stably at one frequency while being powered from a widely varying DC voltage.
The need for such saw-tooth-voltage generating oscillators derives mainly from the requirements of certain switching mode power supplies. Switching mode power supplies (SMPS) are basically DC to DC converters usually operating at kilohertz frequencies. Although the power efficiency of SMPS is generally much higher than that of the older linear shunt and series regulators, it is more difficult in a SMPS to obtain in the output a low value of ripple voltage. To reduce the ultrasonic ripple voltage in a SMPS, a tuned discrete filter is much less expensive and smaller than a broad band ultrasonic filter capable of effecting the same ripple reduction and so it is desirable that the oscillator frequency remain fixed
It is customary for a SMPS to include an oscillator having a sawtooth output voltage.
In a SMPS incorporating the so-called feedforward feature, the rate of rise in the output ramp voltage of the oscillator is required to be kept about proportional to the input voltage. For example, the ramp slope of the oscillator output voltage is doubled for a doubling of the input voltage. This has the effect of regulating the SMPS output voltage against changes in SMPS input voltage as is further explained herein. The SMPS oscillator may in addition be required to operate at very low input voltages. The lowest voltage at which an oscillator can be made operable is called "head-room" and imposes serious limitations on the circuits that may be used to construct the oscillator, e.g. by limiting the number of V.sub.BE drops that the circuit places in series between the supply busses.
The design of a SMPS oscillator with a feedforward feature that will provide a stable frequency with voltage variations over such a broad range has heretofore been frustrated by the fact that circuit techniques that lead to a reliable operation at the high supply voltages to the oscillator, rule out standard techniques for operation at low supply voltages where "head-room" becomes a critical factor.
It is, therefore, an object of the present invention to provide a relaxation oscillator that has a stable frequency while operating from a widely varying supply voltage, especially for incorporation in an integrated-circuit switching mode power supply chip.